by Doug Stanglin, USA TODAY

by Doug Stanglin, USA TODAY

The prosecution in the Oscar Pistorius murder trial questioned the Olympic runner aggressively Wednesday, trying to undermine his efforts to create a sympathetic image over the fatal shooting of his girlfriend, Reeva Steenkamp, who he said he killed while mistaking her for an intruder.

Prosecutors have charged Pistorius, 27, with premeditated murder during an early morning argument on Valentine's Day last year.

Steenkamp, a 29-year-old model, was killed when Pistorius fired his pistol four times through a locked toilet cubicle in the bathroom of his apartment in Pretoria, South Africa.

Under questioning by his defense attorney, Pistorius, known as "Blade Runner" for his use of prosthetics during athletic competition, told the court that when he broke down the cubicle door and found Steenkamp, he tried to keep her from bleeding by using plastic bags and tape.

"I was really trying to stop the bleeding, I was really trying to help Reeva breathe," said Pistorius. He said he tried to hold her head and put pressure on the wound.

"Reeva had already died while I was holding her," he said. "There was nothing more I could do for her."

In an effort to shake Pistorius, prosecutor Gerrie Nel tried to cast him as a cold-blooded killer, asking the paralympist to acknowledge explicitly that he had killed Steenkamp.

Nel also asked the court for permission to show a video of the celebrated Olympic athlete allegedly firing a gun at a range and referring to its deadly power as a "zombie stopper."

Defense lawyer Barry Roux objected to the gun video being shown, saying it was inadmissible character evidence and amounted to a legal "ambush" of the defense. Judge Thokozile Masipa allowed the video to be shown.

PHOTOS: Murder trial of Oscar Pistorius

Earlier, his defense attorney also had Pistorius demonstrate to the court how he could have used a cricket bat to smash into the bullet-marked door, which was set up in the courtroom as evidence.

The prosecution -- suggesting premeditation -- has asserted that the marks on the door indicate that Pistorius had put on his prosthetic legs before going into the bathroom. Pistorius has argued that he was awakened by a noise while sleeping and rushed into the bathroom thinking there was an intruder.

Pistorius also testified about the chaotic moments after he called 911 and tried to get help.

"I asked a policeman if I could wash my hands because the smell of the blood was making me throw up," Pistorius said. "Every time I saw Reeva, I got sick. So I stayed more inside the kitchen. I sat in the pantry against the washing machine."